Ultrafast Interfacial Proton-Coupled Electron Transfer

Petek, Hrvoje; Zhao, Jin

doi:10.1021/cr1001595

Cited by 78 publications

(88 citation statements)

References 160 publications

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“…7A). The wave shape is similar to that for rereduction of Ru IV ¼O 2þ for 2-PO 3 H 2 at pH 5 (0.1 M HOAc∕OAc − ) and the autocatalytic mechanism in [5]. At pH 1 (0.1 M HClO 4 ) at 10 mV∕s, there is clearer evidence for surface loading-dependent, dual pathways for RuðIII → IVÞ oxidation with waves appearing at E p;a ¼ 1.36 and 1.19 V (Fig.…”

supporting

confidence: 59%

“…The disproportionation pathway dominates at Γ∕Γ o ¼ 1. A single, narrow, surface loading-dependent rereduction wave appears with E p;c ¼ 1.16 V at Γ∕Γ o ¼ 1, consistent with autocatalytic comproportionation, [5].…”

mentioning

confidence: 73%

“…concerted electron-proton transfer | metal oxide electrode | surface modification | electrocatalysis | spectroelectrochemistry E lectron transfer reactions involving pH-dependent protoncoupled electron transfer (PCET) couples with a change in proton content between oxidation states, such as quinone/hydroquinone (Q∕H 2 Q) or M¼O∕M-OH∕M-OH 2 oxo/hydroxo/aqua transition metal couples, are often slow at inert electrodes (1)(2)(3)(4)(5). For these couples, the change in protonation state and the requirement to add or lose protons adds to the normal kinetic barrier to electron transfer.…”

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confidence: 99%

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Proton-coupled electron transfer at modified electrodes by multiple pathways

Chen

Vannucci

Concepcion

et al. 2011

Proc. Natl. Acad. Sci. U.S.A.

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In single site water or hydrocarbon oxidation catalysis with polypyridyl Ru complexes such as ½Ru II ðMebimpyÞðbpyÞðH 2 OÞ 2þ [where bpy is 2,2′-bipyridine, and Mebimpy is 2,6-bis(1-methylbenzimidazol-2-yl)pyridine] 2, or its surface-bound analog ½Ru II ðMebimpyÞ ð4,4 0 -bis-methlylenephosphonato-2,2 0 -bipyridineÞðOH 2 Þ 2þ 2-PO 3 H 2 , accessing the reactive states, Ru V ¼O 3þ ∕Ru IV ¼O 2þ , at the electrode interface is typically rate limiting. The higher oxidation states are accessible by proton-coupled electron transfer oxidation of aqua precursors, but access at inert electrodes is kinetically inhibited. The inhibition arises from stepwise mechanisms which impose high energy barriers for 1e − intermediates. Oxidation of the Ru III -OH 2þ or Ru III -OH 2 3þ forms of 2-PO 3 H 2 to Ru IV ¼O 2þ on planar fluoride-doped SnO 2 electrode and in nanostructured films of Sn(IV)-doped In 2 O 3 and TiO 2 has been investigated with a focus on identifying microscopic phenomena. The results provide direct evidence for important roles for the nature of the electrode, temperature, surface coverage, added buffer base, pH, solvent, and solvent H 2 O∕D 2 O isotope effects. In the nonaqueous solvent, propylene carbonate, there is evidence for a role for surface-bound phosphonate groups as proton acceptors.concerted electron-proton transfer | metal oxide electrode | surface modification | electrocatalysis | spectroelectrochemistry E lectron transfer reactions involving pH-dependent protoncoupled electron transfer (PCET) couples with a change in proton content between oxidation states, such as quinone/hydroquinone (Q∕H 2 Q) or M¼O∕M-OH∕M-OH 2 oxo/hydroxo/aqua transition metal couples, are often slow at inert electrodes (1-5). For these couples, the change in protonation state and the requirement to add or lose protons adds to the normal kinetic barrier to electron transfer. The PCET effect arises from the fact that oxidation or reduction at the electrode occurs by electron transfer without a change in proton content. This effect restricts interfacial mechanisms to electron transfer followed by proton transfer (ET-PT) or proton transfer followed by electron transfer (PT-ET). Both involve high-energy intermediates in nonequilibrium protonation states.As an example, oxidation of H 2 Q, H 2 Q − e − → H 2 Q þ• , occurs at E o0 ¼ 1.10 V vs. normal hydrogen electrode (NHE) independent of pH (E o0 is the formal potential) (2). The thermodynamic potential for H 2 Q oxidation at pH 7,. An inert electrode at pH ¼ 7 creates an overpotential of 0.47 V for the initial electron transfer in the ET-PT sequence. Following electron transfer, thermodynamic equilibrium is reached by proton loss from H 2 Q þ• to the surrounding medium at the prevailing pH with ΔE o0 ¼ 1.10 V − 0.059fpH − ðpK a ðH 2 Q þ• Þg with pK a ðH 2 Q þ• Þ ¼ −0.95. Oxidation can also occur by PT-ET with initial loss of a proton from H 2 Q to give HQ − followed by oxidation to HQ• at E o0 ðHQ • ∕HQ − Þ ¼ 0.46 V. For this mechanism, an inhibition to rate arises from the pH depende...

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supporting

confidence: 59%

mentioning

confidence: 73%

mentioning

confidence: 99%

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Proton-coupled electron transfer at modified electrodes by multiple pathways

Chen

Vannucci

Concepcion

et al. 2011

Proc. Natl. Acad. Sci. U.S.A.

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“…22,23 TR-2PP has been applied to the spectroscopy and dynamics of single crystal rutile TiO 2 (110) surfaces under ultrahigh vacuum (UHV) conditions in the contexts of both photocatalysis and dye sensitized solar cells. [24][25][26][27][28][29][30][31][32][33] The well known surface preparation, properties, and chemistry make rutile TiO 2 (110) well suited for studies of elementary surface and bulk charge carrier processes triggered by photoexcitation in metal oxides. 4,6,7 Previous TR-2PP experiments on clean and protic solvent covered TiO 2 (110) surfaces with 400 nm (3.1 eV) excitation focused on the surface electronic structure.…”

Section: Introductionmentioning

confidence: 99%

Ultrafast multiphoton pump-probe photoemission excitation pathways in rutileTiO2(110)

et al. 2015

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We investigate the spectroscopy and photoinduced electron dynamics within the conduction band of reduced rutile TiO 2 (110) surface by multiphoton photoemission (mPP) spectroscopy with wavelength tunable ultrafast (~20 fs) laser pulse excitation. Tuning the mPP photon excitation energy between 2.9 and 4.6 eV reveals a nearly degenerate pair of new unoccupied states located at 2.73±0.05 and 2.85±0.05 eV above the Fermi level, which can be analyzed through the polarization and sample azimuthal orientation dependence of the mPP spectra. Based on the calculated electronic structure and optical transition moments, as well as related spectroscopic evidence, we assign these resonances to transitions between Ti 3d-bands of nominally t 2g and e g symmetry, which are split by crystal-field. The initial states for the optical transition are the reduced Ti 3+ states of t 2g symmetry populated by formation oxygen vacancy defects, which exist within the band gap of TiO 2 . Furthermore, we studied the electron dynamics within the conduction band of TiO 2 by three-dimensional (3D) time-resolved pump-probe interferometric mPP measurements. The spectroscopic and time-resolved studies reveal competition between 2PP and 3PP processes where the t 2g -e g transitions in the 2PP process saturate, and are overtaken by the 3PP process initiated by the band gap excitation from the valence band of TiO 2 .

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“…It would give access to the coupled motion of electrons and nuclei during molecular dynamics (50). Time-resolved x-ray probing of, in particular, the occupied valence electronic structure gives direct access to the evolution of chemical bonding.…”

Section: Introductionmentioning

confidence: 99%

Electronic structure in real time: mapping valence electron rearrangements during chemical reactions

Wernet

2011

Phys. Chem. Chem. Phys.

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The interest in following the evolution of the valence electronic structure of atoms and molecules during chemical reactions on a femtosecond time scale is discussed. By explicitly mapping the occupied part of the electronic structure with femtosecond pump-probe schemes one essentially follows the electrons making the bonds while the bonds change. This holds the key to unprecedented insight into chemical bonding in short-lived intermediates and reveals the coupled motion of electrons and nuclei. Examples from the recent literature on small molecules and anionic clusters in the gas phase and on atoms and molecules on surfaces using lab-based femtosecond laser methods are used to demonstrate the case. They highlight how the evolution of the valence electronic structure can be probed with time-resolved photoelectron spectroscopy with ultraviolet (UV) probe photon energies of up to 6 eV. It is shown how new insight can be gained by extending the probing wavelength into the vacuumultraviolet (VUV) region to photon energies of 20 eV and more by accessing the whole occupied valence electronic structure with time-resolved VUV photoelectron spectroscopy.Finally, the importance of soft x-ray free-electron lasers with probe photon energies of several hundred eV and femtosecond pulses and in particular the key role of femtosecond timeresolved soft x-ray emission spectroscopy or resonant inelastic x-ray scattering for mapping the electronic structure during chemical reactions is discussed.2

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Ultrafast Interfacial Proton-Coupled Electron Transfer

Cited by 78 publications

References 160 publications

Proton-coupled electron transfer at modified electrodes by multiple pathways

Proton-coupled electron transfer at modified electrodes by multiple pathways

Ultrafast multiphoton pump-probe photoemission excitation pathways in rutileTiO2(110)

Electronic structure in real time: mapping valence electron rearrangements during chemical reactions

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